Atmospheric pressure ionization (API) methods have been widely used in mass spectrometry applications because they can be utilized for a wide range of chemical and biological samples. Ionization of a gaseous analyte sample at atmospheric pressure has advantages such as simplicity and accessibility during the operation. Thus, mass spectrometer systems are designed such that a sample, ionized at atmospheric pressure, is transmitted through a mass spectrometer sample input interface (hereinafter “mass spectrometer interface” or “interface”) into the mass spectrometer for analysis.
Mass spectrometers typically operate at pressures much lower than atmospheric pressure, typically 10−4 to 10−9 torr. Such pressures are generally regarded, and referred to, as vacuum pressure. Thus, one design objective of a mass spectrometer interface is to accommodate this orders-of-magnitude difference in pressure. The interface facilitates the evacuation of the ionized gaseous sample down to the mass spectrometer's operating pressure as it directs the sample into the mass spectrometer. As a consequence, a large portion of the ions generated at atmospheric pressure in the sample are lost during the process of evacuation and transmission. This loss potentially can be a drawback, in that it tends to reduce the sensitivity of the mass spectrometer.
In many mass spectrometer systems, analyte sample solutions are atomized or sprayed into a mist of fine droplets, and then ionized, to impart electrical charge on the droplets. These charged droplets undergo a desolvation process, and become single or multiple charged gaseous ions. However, some of the droplets survive the desolvation and enter the mass spectrometer's vacuum chamber. Incompletely desolvated droplets of analyte solution in a mass spectrometer can reach the mass spectrometer's detector, and cause undesirable noise signals, thereby reducing the sensitivity of the mass spectrometer.
Therefore, to maximize sensitivity of the analysis by the mass spectrometer, the interface should be designed (i) to minimize sample loss or otherwise operate effectively despite the sample loss, and (ii) to maximize desolvation and otherwise minimize or eliminate noise.